CholmodDecomposition now has explicit variants. These variants will allow to provide access to the underlying factors.

 - remove outdated/deprecated code
 - improve a bit the documentation

* change Sparse* specialization's signatures from <..., int Backend> to <..., typename Backend>. Update SparseExtra accordingly to use structs instead of the SparseBackend enum.
* add SparseLDLT Cholmod specialization
* for Cholmod and UmfPack, SparseLU, SparseLLT and SparseLDLT now use ei_solve_retval and have the new solve() method (to be closer to the 3.0 API).

* fix doc

 - all what's not in unsupported/ is considered stable API
    (except internal stuff e.g. expression templates).

(see the ML for details)